Typically, semiconductor substrates have a silicon base and dielectric layers containing multiple trenches arranged to form a pattern of circuit interconnects within the dielectric layer. These trench patterns have either a damascene structure or dual damascene structure. In addition, typically one to as many as three or more capping layers coat the trench patterned dielectric layer with a barrier layer covering the capping layer or capping layers. Finally, a metal layer covers the barrier layer and fills the patterned trenches. The metal layer forms circuit interconnects that connect dielectric regions and form an integrated circuit. Typically, these circuits contain copper for horizontal interconnections and tungsten for vertical interconnections.
For the advanced technical nodes with line widths of 10 nm and below, cobalt-containing films are being implemented on top of Ti/TiN or Ta/TaN barrier layers for the first metal interconnects. These cobalt-containing interconnects have a high depth to width aspect ratios that renders copper seed and subsequent copper electrodeposition. This enables a much thinner copper seed layer deposition, which improves the trench Cu electroplating process and avoid void formation. In addition, cobalt can replace copper and form entire interconnects for the first metal interconnects. Finally, cobalt can replace tungsten for trenches that connect multiple transistors. All these new processes require CMP to achieve planarity to the desired targeted thickness, dishing, defectivity and selectivities.
For slurries that involve in these processes, they need to have special chemistries to handle the peculiarities of cobalt films. First, cobalt is prone to oxidation and corrosion, as it is an active metal. Cobalt's reduction potential (−0.28 V vs. SHE) makes it an easy corrosion target when in contact with other nobler metals such as copper (+0.34 V SHE). Since first layer interconnects contain cobalt in combination with the more noble copper, galvanic corrosion is of particular concern during cobalt polishing of this layer. Second, corrosion defects and the resulting metal roughness after CMP are major challenges in developing successful cobalt CMP slurry.
There is a demand for a polishing method that can polish cobalt in multiple films without excessive corrosion. Furthermore, there is a demand for a cobalt method that can polish cobalt into a planar surface with little surface roughness.